Throughout this specification, the terms “transmit signals” and “receive signal” refer to signals that are transmitted from, and received by, the antenna.
The rotating antenna is suitable for use as part of a vehicle positioning and tracking radar system in combination with a coded modulated transponder which returns or transmits a receive signal to the antenna having a different polarisation to the transmit signals. The term “coded modulated transponder” refers to any transponder which can superimpose a temporal code on the incident transmit signals using a chosen modulation scheme. The chosen modulation scheme can be based on amplitude or phase modulation, for example.
The transponder is preferably passive in the sense that it does not require frequent battery changes. The transponder is also preferably retro-reflective so that the incident transmit signals are directed back towards the antenna for a broad range of incidence angles. However, it will be appreciated that the transponder can also be active in which case it will require a permanent power source. A suitable transponder is a retro-reflective array transponder described in WO 00/59068 (Isis Innovation Limited) adapted so that it alters the polarisation of the transmit signals.
The radar system is particularly suitable for tracking the position of marine vessels or land vehicles that operate on a fixed plane. The antenna can be mounted on the marine vessel and the transponder can be positioned on a fixed platform such as a harbour wall, drilling rig or buoy, for example. As the marine vessel approaches the fixed platform, the antenna will illuminate the transponder and the radar system can use the receive signal to determine the range, bearing and velocity of the marine vessel relative to the transponder. The radar system can provide information for the manual or automatic control of the marine vessel so that it can be held a predetermined distance from the fixed platform. If the radar system includes a second coded modulated transponder (see below) then the marine vessel can also be held at a predetermined orientation relative to the fixed platform.
It will be readily appreciated that the radar system can be used in other ways. For example, the transponder can be mounted on a marine vessel or a land vehicle and the location of the rotating antenna can be fixed. As long as the marine vessel or land vehicle is within the predetermined area that is illuminated by the rotating antenna, the radar system can use the receive signal from the transponder to determine the range, bearing and velocity of the marine vessel or land vehicle relative to the antenna. The position of marine vessel or the land vehicle can then be noted at regular intervals so that its movement within the predetermined area can be tracked.
The transmit signals are preferably vertically polarised and the receive signal is preferably horizontally polarised, or vice versa. The cross-polarisation of the transmit signals and the receive signal prevents power leakage between the transmit and receive feeds at the antenna which would otherwise be significant in an FMCW radar system.
The radar system is preferably capable of simultaneously processing the receive signal from two or more transponders. The radar system can therefore include a second coded modulated transponder. Further coded modulated transponders can be added depending on the operational requirements of the radar system. The transponders preferably operate at different modulation frequencies and superimpose different temporal codes on the incident transmit signals so that the radar system can easily distinguish between the receive signal from one transponder and the receive signal from another transponder.
The capacity to distinguish different temporal codes from transponders co-located at the same azimuthal position also helps the radar system to discriminate the receive signal from the background clutter and multi-path reflections while still taking accurate bearing measurements to each transponder. Monopulse is a standard radar technique applied to servoed antenna signals that enables the signal processing to lock on to a target during operation. Applying monopulse techniques to a rotating antenna has been found to give an improvement in the amount of information that can be obtained in a single scan and allows the radar system to obtain a better bearing estimate. However, monopulse techniques do not work well if two targets are close together. Because the radar system of the present invention provides separate signals from each transponder, this limitation is removed and monopulse techniques can still be used to accurately determine a bearing estimate to each transponder on a “per sample” basis whether they are co-located or not, whilst still enabling the temporal coding to reduce the risk that background clutter will seriously degrade the achievable angular resolution.
An optical encoder can be used to determine the angular position of the antenna.